Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S. Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, synthetic diamond material can be deposited.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for synthetic diamond film growth using a CVD process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
A useful overview article by Silva et al. summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see “Microwave engineering of plasma-assisted CVD reactors for diamond deposition” J. Phys.: Condens. Matter, Vol. 21, No. 36 (2009) 364202). Having regard to the patent literature, U.S. Pat. No. 6,645,343 (Fraunhofer) discloses an example of a microwave plasma reactor configured for diamond film growth via a chemical vapour deposition process. The reactor described therein comprises a cylindrical plasma chamber with a substrate holder mounted on a base thereof. A cooling device is provided below the substrate holder for controlling the temperature of a substrate on the substrate holder. Furthermore, a gas inlet and a gas outlet are provided in the base of the plasma chamber for supplying and removing process gases. A microwave generator is coupled to the plasma chamber via a high-frequency coaxial line which is subdivided at its delivery end above the plasma chamber and directed at the periphery of the plasma chamber to an essentially ring-shaped microwave window in the form of a quartz ring mounted in a side wall of the plasma chamber.
Using microwave plasma reactors such as those disclosed in the prior art it is possible to grow polycrystalline diamond wafers by chemical vapour deposition on a suitable substrate such as a silicon wafer or a carbide forming refractory metal disk. Such polycrystalline CVD diamond wafers are generally opaque in their as-grown form but can be made transparent by polishing opposing faces of the wafers to produce transparent polycrystalline diamond windows for optical applications.
Diamond material is useful as a heat spreading component as it has a high thermal conductivity. For example, one such application is as a heat spreading substrate in a disk laser as illustrated schematically in FIG. 1. A disk laser comprises a heat spreading substrate S on which a thin disk of laser gain material LGM is disposed. The thin disk is also often called an active mirror because it acts as a mirror with laser gain. The heat spreading substrate may be subjected to a coolant C for extracting and removing heat therefrom. An output coupler O is positioned opposite the active mirror to form an optical cavity OC. The active mirror is pumped with, for example, a diode laser DL and high powered laser light LL is emitted through the output coupler.
It is known to use a polycrystalline CVD synthetic diamond wafer as a heat spreading substrate for mounting the active mirror of a disk laser. Diamond material has been found to be useful in such an application because of its extremely high thermal conductivity. Furthermore, diamond material has a very low thermal expansion coefficient such that thermal distortion is low.
The thermal performance of a polycrystalline CVD synthetic diamond wafer is dependent on the physical dimensions of the wafer (diameter and thickness) and the quality of the diamond material forming the wafer. For example, a thick, large area wafer will tend to have better heat spreading functionality than a thin, small area wafer. Furthermore, it is known that thermal conductivity is affected by grain size, impurities and/or defects such as non-diamond carbon which are incorporated into the diamond material during growth. In addition, material quality is intimately linked with wafer geometry and growth rate. For example, growing wafers to increased thickness tends to increase the rate at which impurities and/or defects are incorporated into a polycrystalline CVD synthetic diamond wafer. Furthermore, growing wafers to increased diameter tends to increase the rate at which impurities and/or defects are incorporated into a polycrystalline CVD synthetic diamond wafer, particularly at a periphery of the wafers. Further still, growing wafers at increased growth rate tends to increase the rate at which impurities and/or defects are incorporated into a polycrystalline CVD synthetic diamond wafer. In addition, growing wafers to increased thickness, diameter, and/or growth rate can also lead to problems of wafer cracking during the synthesis process.
For applications such as high performance disk lasers, it is desirable to provide polycrystalline CVD synthetic diamond wafers having a diameter of 20 mm, a thickness of at least 2 mm, and a thermal conductivity of at least 2000 Wm−1K−1. Typical thermal-grades of polycrystalline CVD synthetic diamond wafers tend to have a thermal conductivity of less than 2000 Wm−1K−1. Accordingly, for such high performance thermal applications, to date higher quality dielectric-grade or optical-grade polycrystalline CVD synthetic diamond wafers have been utilized. However, such higher grades of polycrystalline CVD synthetic diamond material are generally grown at lower growth rates in order to achieve better quality material leading to increased expense.
In light of the above, it is an aim of certain embodiments of the present invention to provide a lower cost microwave plasma CVD diamond synthesis process in order to fabricate thick polycrystalline CVD synthetic diamond wafers having high thermal conductivity for heat spreading applications such as in disk lasers.